1. Field of the Invention
The present invention relates to a steering system for vehicles. Particularly, it relates to a steering system for a vehicle having front and rear wheels, in which the rear wheels as well as the front wheels are steerable by steering a means of wheel.
2. Description of Relevant Art
There have been proposed steering systems for vehicles, such as in Japanese Patent Application Nos. 56-118698 and 57-47743, in which a rear wheel steering sub-system is provided with a crank member, such as a pivotable link or an off-centered pin, capable of a circular motion, and with a linkage member for transmitting an angular displacement taken out in accordance with the steering of a steering wheel from a front wheel steering sub-system to the crank member, to thereby rotate the crank member. A transverse linear displacement subjected substantially to a sine function is obtained therefrom, to thereby steer both rear wheels concurrently with the front wheels.
With such a steering system, for a vehicle having front and rear wheels, the rear wheels are able to be steered in the same direction as the front wheels for relatively small steering angles of a steering wheel and in the opposite direction thereto for relatively large steering angles thereof. Accordingly, there is provided favorable vehicle maneuverability when travelling at relatively high speeds, as well as favorable vehicle turning characteristics due to relatively small turning radii possible with large steering angles, so as to facilitate implementing a U-turn and steering in and out of a parking space.
However, with such a conventional steering system, the relation between the steered angle of both rear wheels and the steering angle of a steering wheel is substantially bound to a simple sine function, thus being unable to sufficiently accommodate the peculiarity of the rear wheel steering.
For example, the maximum steered angle of the rear wheels, as steered in the same direction as the front wheels, becomes the same as that when the rear wheels are steered in the opposite direction to the front wheels, while the latter is generally desired to be larger.
Moreover, for the rear wheel steering, it is fundamentally desirable to provide a voluntary selection of the steered angle ratio as well as the steering direction of the rear wheels relative to the front wheels in accordance with various steering situations such as when parking, cruising at relatively low speeds along narrow curves and travelling at considerably high speeds, or to suit the taste of individual drivers.
Further, even in a steering system with which both front and rear wheels are steerable in proportion to the steering angle of a steering wheel, it is also desirable to provide voluntary selection of the rear wheels steering mode, such as in accordance with the steering situation or the taste of the driver.
Incidentally, it is known with respect to the rear wheels of a turning vehicle, that the wheel at the outer side with respect to the turning has a greater effect on the maneuverability of the vehicle than the other one at the inner side.
In this connection, for conventional steering systems, still more desired is the possibility of steering either rear wheel, as located at the outer side in each vehicle-turning operation, by larger angles than the other one at the inner side.
The present invention has been achieved to effectively satisfy such desires for conventional steering systems for vehicles.